1. Field of the Invention
The present invention relates to an imaging apparatus and a control method for the same, and relates in particular to an auto focus (hereinafter abbreviated as “AF”) control based on a photoelectric conversion signal output from an imaging element.
2. Description of the Related Art
One focus detection method performed by an imaging apparatus is an imaging plane phase-difference type method that detects a focus state using focus detecting pixels formed in an imaging element. Another focus detection method performed by an imaging apparatus is a contrast type method that detects a focus state using a contrast evaluation value based on a shot imaging signal output from an imaging element.
Japanese Patent Laid-Open No. 2013-025246 discloses an imaging plane phase-difference type imaging apparatus that includes a contrast evaluating unit and a correlation calculating unit, and compares two absolute values of focus evaluation ranges obtained from these units to thereby determine the focus evaluation value of an object based on the comparison result. The contrast evaluating unit determines a contrast focus position based on the contrast evaluation value of a signal obtained by shift summation of imaging signals from different pupil areas. Thus, a focus position can be specified without actually driving a focus lens for AF control.
Japanese Patent No. 3592147 discloses a solid-state image pickup device in which photoelectric conversion cells for converting an optical image formed by an optical system into an electrical signal are two-dimensionally laid out, and at least part of the photoelectric conversion cell group is configured to output a signal for distance measurement.
Japanese Patent No. 5274299 discloses a technique for calculating the focus state (defocus amount) of the shooting lens by using a first image obtained from focus detecting pixels and a second image obtained from imaging pixels, while reducing the degree of the degradation of the image quality.
However, Japanese Patent Laid-Open No. 2013-025246 does not describe an imaging element that two-dimensionally lays out pixels, which allows a relative position between a microlens and a photoelectric conversion unit to be biased, as disclosed in Japanese Patent No. 3592147. If the focus detection of the contrast evaluating unit in Japanese Patent Laid-Open No. 2013-025246 is performed with the imaging element disclosed in Japanese Patent No. 3592147, the precision for the focus detection is reduced because a pair of focus detecting pixels is spatially spaced apart from each other.
Also, the device disclosed in Japanese Patent No. 5274299 detects the defocus amount based on the difference of the center of the gravity between the first image obtained from the focus detecting pixels and the second image obtained from imaging pixels. Therefore, the precision of the focus detection is reduced compared to the focus detection of the contrast evaluating unit as disclosed in Japanese Patent Laid-Open No. 2013-025246.